Plant for recovering degraded heat transfer oil from a solar thermal facility and associated recovery method

ABSTRACT

The invention can be used to improve the yield of degraded heat transfer oil ( 1 ) recovered from a solar thermal facility, comprising the successive separation of heavy components ( 4 ) (ortho-, meta- and para-terphenyl) and light components ( 7 ) (phenol and benzene). The plant comprises: an air-cooled cooler ( 2 ) which cools the degraded heat transfer oil ( 1 ) to obtain a cooled oil ( 12 ); a distillation column ( 3 ) which separates the heavy components ( 4 ) and a vapour ( 15 ) of light components ( 7 ) plus oil from the cooled heat transfer oil ( 12 ); and a rectifier ( 6 ) which separates the light components ( 7 ) and the regenerated oil ( 17 ) from the vapour ( 15 ). The method comprises the cooling of the degenerated heat transfer oil ( 1 ) in the air-cooled cooler ( 2 ), the separation of the heavy components ( 4 ) in the distillation column ( 3 ), and the separation of the light components ( 7 ) and the regenerated oil ( 17 ) in the rectifier ( 6 ).

OBJECT OF THE INVENTION

The present invention is applicable to the technical field of solar energy capture in parabolic trough collector plants, specifically in the regeneration of the heat transfer oil used in said plants.

The object of the invention is a plant for regenerating degraded heat transfer oil for solar thermal facilities and a method for carrying out said regeneration.

BACKGROUND OF THE INVENTION

Parabolic trough collector solar plants normally use a heat transfer fluid that can be both thermal oil and water, in direct steam generation collectors. In the case of collectors that use oil as a heat transfer fluid, a thermal oil is heated in the collectors which, after passing through a heat exchanger, allows the production of pressurized steam that is subsequently expanded in a conventional Rankine cycle turbine.

There are a large variety of compounds which are used as thermal oil for solar plants, each having specific characteristics. Specifically, one of these oils is composed of a eutectic mixture of biphenyl and diphenyl oxide (marketed under the name Dowtherm A or Therminol VP1). The properties of this type of oil make it especially suited for heat transfer within a temperature range of up to 400° C.

The thermal stability of a fluid is determined by its composition. When the temperature of the oil reaches the working values of the solar thermal plant (temperatures of up to 400° C.), the molecular bonds of the fluid structure break to form two large types of degradation products: light compounds and heavy compounds.

In the specific case of the aforementioned oil, it undergoes slow decomposition into light components (low boilers), essentially benzene and phenol, and heavy components (high boilers), essentially o-terphenyl, m-terphenyl, p-terphenyl and 2-phenoxybiphenyl. The concentration of these oil degeneration products should not exceed certain limits due to the fact that light components, having a high vapour pressure, increase the pressure in the system and can cause cavitation in the pumps, while the heavy components limit the efficiency of the heat transfer as, among other things, they reduce the specific heat of the heat transfer oil and increase its viscosity.

In order to avoid accumulation, said heavy components and light components must be periodically removed from the facility, as they modify the properties of the heat transfer oil.

It has been experimentally verified that simple flash distillation (single-step distillation) followed by partial condensation of the vapour current generated in the distiller is unable to effectively regenerate the oil. The separation of significant amounts of heavy components is necessarily associated with considerable oil losses. Thus, for example, the elimination of 70% of the heavy components entails oil losses of approximately 30%.

The technical problem being addressed consists of describing a thermal oil regeneration plant capable of minimising oil degradation and oil losses and improving regeneration performance, as the price of said oil is considerably high.

DESCRIPTION OF THE INVENTION

The present invention solves the addressed problem by means of a plant for regenerating the heat transfer oil circulating through the collectors of a solar thermal plant, according to a first object of the invention, and a method for carrying out said regeneration, according to a second object of the invention.

The regeneration plant comprises:

-   a distillation column (stripper) to separate the heavy components; -   a boiler; -   a rectifier for separating the light components; -   an air condenser; -   a backflow tank; -   a first pump and second pump.

The plant of the invention can additionally comprise an air cooler. In such case, said air cooler is the first element of the plant. The degraded oil reaches the air cooler, at a high temperature, from the solar facility. Preferably, the temperature at the inlet of the air cooler is comprised between 295° C. and 310° C. The temperature at the outlet of the air cooler is preferably comprised between 95° C. and 110° C. Cooled oil is obtained at the outlet of the air cooler.

Said temperature of between 95° C. and 110° C., preferably 100° C., is sufficiently low to prevent heavy components from being carried away by the vapour that flows out of the top of the distillation column, with respect to the case of not using pre-cooling, which substantially improves regeneration performance. Cooling the degraded oil before introducing it in the distillation column improves the performance of said distillation column.

In the absence of an air cooler, the degraded oil is introduced directly in the distillation column. In the event of having an air cooler, at the outlet of said air cooler the cooled oil is introduced in the distillation column. The distillation column comprises a column of first perforated plates made of carbon steel, and a boiler. The oil enters the distillation column through the upper part thereof and is heated in said distillation column by the boiler.

The harmful heavy components (o, m and p-terphenyls) are separated into tails by gravity, together with a small amount of oil, while practically all the oil and light components pass through the successive first plates until they reach the upper part of the distillation column, called the head, where they abandon the distillation column in the form of vapour.

Preferably, the boiler is a thermosyphon-type heat exchanger that works by means of natural circulation of the hot oil from the solar facility.

The vapour, substantially free of heavy components, which abandons the distillation column through the head thereof, accesses the rectifier. The rectifier is a distillation column having a plurality of second perforated plates made of carbon steel. The regenerated oil is recovered in the lower part of the rectifier and the light components are recovered in the upper part thereof, which are subsequently condensed in the air cooler and conveyed toward a backflow tank.

Additionally, the invention includes a first pump and a second pump. The first pump conveys the regenerated oil toward its working fluid circuit in the solar thermal collectors. The second pump conveys the light components from the backflow tank toward the rectifier. The circulation enabled by the second pump is necessary because the pressure in the tank is lower than in the rectifier.

By means of the invention, regenerated oil with 99.4% purity is obtained, compared to the initial 94.3% purity of the oil prior to regeneration.

DESCRIPTION OF THE DRAWINGS

In order to complement the description being made and with the object of helping to better understand the characteristics of the invention, according to a preferred practical embodiment thereof, a set of drawings are included wherein the following is represented in an illustrative and non-limiting manner:

FIG. 1 shows a schematic view of the plant according to the invention.

PREFERRED EMBODIMENT OF THE INVENTION

Following is a description of a preferred embodiment of the invention, with the aid of the single FIG. 1.

The plant for regenerating degraded heat transfer oil (1) from a solar thermal facility according to the invention is used to regenerate the oil used in said solar thermal facility, which is a eutectic mixture of biphenyl (26.5%) and diphenyl oxide (73.5%). Table 1 shows the characteristics of this oil, specifically of Dowtherm A.

TABLE 1 Properties of Dowtherm A thermal oil Vapour Specific Thermal Temperature pressure Viscosity heat conductivity Density ° C. (bar) (mPa · s) (kJ/kg/K) (W/m/K) (kg/m³) 12 0 5.52 1.55 0.14 1065.9 15 0 5 1.558 0.1395 1063.5 20 0 4.29 1.573 0.1387 1059.6 30 0 3.25 1.601 0.1371 1051.7 40 0 2.56 1.63 0.1355 1043.8 50 0 2.07 1.658 0.1339 1035.8 60 0 1.72 1.687 0.1323 1027.8 70 0 1.46 1.715 0.1307 1019.7 80 0 1.25 1.744 0.1291 1011.5 90 0 1.09 1.772 0.1275 1003.2 100 0.01 0.97 1.8 0.1259 994.9 110 0.01 0.86 1.828 0.1243 986.5 120 0.01 0.77 1.856 0.1227 978.1 130 0.02 0.7 1.884 0.1211 969.5 140 0.03 0.64 1.912 0.1195 960.9 150 0.05 0.58 1.94 0.1179 952.2 160 0.07 0.53 1.968 0.1163 943.4 170 0.09 0.49 1.996 0.1147 934.5 180 0.13 0.46 2.023 0.1131 925.5 190 0.18 0.42 2.051 0.1115 916.4 200 0.24 0.39 2.079 0.1099 907.1 210 0.32 0.37 2.107 0.1083 897.8 220 0.42 0.34 2.134 0.1067 888.3 230 0.54 0.32 2.162 0.1051 878.7 240 0.69 0.3 2.19 0.1035 868.9 250 0.87 0.28 2.218 0.1019 859 260 1.08 0.27 2.245 0.1003 849 270 1.33 0.25 2.273 0.0987 838.7 280 1.63 0.24 2.302 0.0971 828.3 290 1.98 0.22 2.33 0.0955 817.7 300 2.38 0.21 2.359 0.0939 806.8 310 2.84 0.2 2.388 0.0923 795.8 320 3.37 0.19 2.417 0.0907 784.4 330 3.96 0.18 2.448 0.0891 772.8 340 4.64 0.17 2.479 0.0875 760.9 350 5.39 0.16 2.511 0.0859 748.6 360 6.24 0.15 2.544 0.0843 735.9 370 7.18 0.15 2.579 0.0827 722.8 380 8.22 0.14 2.616 0.0811 709.2 390 9.37 0.13 2.657 0.0795 695 400 10.64 0.13 2.701 0.0779 680.2

The initial composition by weight of the degraded heat transfer oil (1) is shown below. The initial conditions of said degraded heat transfer oil (1) are 302° C. and 33 bar:

Benzene 0.4% Phenol 1.4% Ortho-Terphenyl 2.9% Meta-Terphenyl 0.5% Para-Terphenyl 0.5% Dowtherm A 94.3%

Table 2 below shows the composition of the currents flowing in and out of the plant of the invention.

TABLE 2 Composition of the currents flowing in and out of the plant of the invention. Oil to be Heavy Light Regenerated regenerated components components oil Flow (kg/h) 6,500 253.5 117 6129.5 Benzene 0.4 — 24.1 173 ppm Phenol 1.4 — 60.3 0.3 Ortho- 2.9 68.4 — 0.3 Terphenyl Meta- 0.5 12.1 — 152 ppm Terphenyl Para- 0.5 12.2 — 115 ppm Terphenyl Dowtherm A 94.3 7.2 15.6 99.4 Temperature 302° C. 351 138 275 (° C.) Pressure 33 1.5 1.5 1.5 (bar)

As shown in Table 2, the facility of the invention is appropriate for treating a volume of degraded heat transfer oil (1) of 6500 kg/h, which allows processing of the total amount of oil used in the solar thermal plant, 1300 tonnes, in a total of 200 hours. The indicated volumes can have variations of up to +−10% without significantly affecting the performance of the plant of the invention.

The plant comprises the following elements:

-   an air cooler (2); -   a distillation column (3) for separating the heavy components (4); -   a boiler (5); -   a rectifier (6) for separating the light components (7); -   an air condenser (8); -   a backflow tank (9); -   a first pump (10) and second pump (11).

The air cooler (2) receives the degraded heat transfer oil (1) from the solar thermal facility and lowers its temperature from an inlet temperature of 302° C. to an outlet temperature of 100° C. The air cooler (2) incorporates means for controlling outlet temperature (not shown), which comprise a frequency converter or various individually controlled fans. The air cooler (2) also incorporates inlet and outlet temperature meters and a flow meter.

The output product of the air cooler (2) is cooled oil (12), which is introduced in the distillation column (3), which comprises a column of first perforated plates (13) made of carbon steel, and a boiler (5). The cooled oil (12) enters the distillation column (3) through the upper part thereof and is heated in said distillation column (3) by the boiler (5), which is a thermosyphon-type heat exchanger, in order to evaporate the products in the lower part of the distillation column (3) by means of a current of the hot oil coming from the solar thermal facility.

The boiler (5) incorporates temperature and flow meters and a flow control valve (not shown).

The harmful heavy components (4) (o, m and p-terphenyls) are separated into tails by gravity, together with a small amount of oil, while the rest of the oil and light components penetrate the distillation column (3) until reaching the head (14), where they abandon said distillation column (3) in the form of vapour (15).

The distillation column (3) incorporates a pressure transmitter at the inlet and a pressure and temperature transmitter at the outlet. It also incorporates a plurality of temperature transmitters distributed throughout the interior thereof, one every three first plates (13), and one temperature transmitter in the lower part thereof, four level switches and an automatic valve at the outlet of the heavy components (4).

The distillation column (3) incorporates an electric resistance (not shown) in the lower part thereof for the purpose of maintaining the oil in a liquid state (250° C.) in the event that the power supply is interrupted.

The vapour (15) free of heavy components (4) abandons the distillation column (3) through the head (14) and accesses the rectifier (6), which is a distillation column having a plurality of second perforated plates (16) made of carbon steel. The regenerated oil (17) is recovered in the lower part of the rectifier (6) and the light components (7) are collected in the upper part thereof, which are condensed in the air condenser (8) and conveyed toward a backflow tank (9).

The rectifier (6) incorporates pressure and temperature transmitters at the outlet, temperature transmitters distributed throughout the interior of the rectifier (6), an additional temperature transmitter in the lower part and level switches, in addition to incorporating an electric resistance in the lower part thereof for the purpose of maintaining the fluid in a liquid state (30° C.) in the event that the power supply is interrupted.

The air condenser (8) is equipped with means for controlling outlet temperature, preferably a frequency converter or a plurality of independently controlled fans.

The accessory elements, such as valves, auxiliary equipment and instruments, have been described in detail, although these may vary in the final construction.

Additionally, the invention incorporates a first pump (10), which circulates the regenerated oil (17) back to the solar thermal facility, and a second pump (11) that recirculates the light components (7) from the backflow tank (9) to the rectifier (6). 

1-26. (canceled)
 27. A plant for recovering degraded heat transfer oil from a solar thermal facility, the plant comprising: a distillation column for separating heavy components from the degraded oil, the distillation column comprising an upper part and a lower part, the upper part having: a first inlet for receiving the degraded oil, and a head having a first outlet for expelling light component vapour; a rectifier for receiving the vapour, the rectifier having: an upper part having a second outlet for expelling light components; and a lower part having a third outlet for expelling regenerated oil; and a fourth outlet for letting the heavy components out.
 28. The plant of claim 27, wherein the distillation column comprises a column of perforated first plates.
 29. The plant of claim 27, wherein the distillation column comprises: a pressure transmitter at the first inlet, a pressure and temperature transmitter at the first outlet, a plurality of temperature transmitters distributed throughout the interior, level switches, an automatic valve at the fourth outlet, and an electric resistance in the lower part for maintaining the oil in a liquid state.
 30. The plant of claim 28, wherein the distillation column further comprises temperature transmitters in the interior which are distributed one every three first plates.
 31. The plant of claim 27, additionally comprising a boiler configured so as to use hot heat transfer oil coming from the solar thermal facility for evaporating the oil circulating in the lower part of the distillation column
 32. The plant of claim 31, wherein the boiler additionally comprises temperature and flow meters and a flow control valve.
 33. The plant of claim 27, wherein the rectifier (6) comprises a plurality of perforated second plates.
 34. The plant of claim 27, wherein the rectifier comprises: pressure and temperature transmitters at the second and/or third outlets, temperature transmitters distributed throughout the interior, a temperature transmitter in the lower part, level switches, and an electric resistance in the lower part for maintaining the fluid in a liquid state.
 35. The plant of claim 27, additionally comprising an air condenser for condensing the light components collected in the upper part of the rectifier.
 36. The plant of claim 35, wherein the air condenser comprises a frequency converter for controlling outlet temperature.
 37. The plant of claim 35, wherein the air condenser comprises a plurality of independently controlled fans for controlling outlet temperature.
 38. The plant of claim 27, additionally comprising a first pump for circulating the regenerated oil back to the solar thermal facility.
 39. The plant of claim 27, further comprising: a backflow tank for receiving the light components condensed in the air condenser, and a second pump for recirculating the light components from the backflow tank to the rectifier.
 40. The plant of claim 27, further comprising an air cooler for cooling the degraded heat transfer oil from an inlet temperature to a cooled oil outlet temperature.
 41. The plant of claim 27, wherein the air cooler comprises a frequency converter for controlling the outlet temperature.
 42. The plant of claim 27, wherein the air cooler comprises a plurality of independently controlled fans for controlling the outlet temperature.
 43. The plant of claim 27, wherein the air cooler comprises inlet and outlet temperature meters and a flow meter.
 44. A process for recovering degraded heat transfer oil from a solar thermal facility, comprising the following steps: separation of the degraded oil into heavy components and light components plus oil vapour, using a distillation column; and separation of the oil vapour into regenerated oil and light components, using a rectifier.
 45. The process of claim 44, further comprising a step of cooling the degraded oil in an air cooler, prior to separation in the distillation column.
 46. The process of claim 45, wherein: the air cooler receives the degraded heat transfer oil from the solar thermal facility and lowers its temperature from an inlet temperature to an outlet temperature; the oil cooled by the air cooler is introduced in the distillation column through the upper part thereof and is heated by a boiler for evaporating the products in the lower part of the distillation column by means of a current of said hot oil coming from the solar thermal facility; the heavy components are separated into tails by gravity, together with a small amount of oil, while the rest of the oil and light components penetrate the distillation column until reaching the head, wherethrough they abandon said distillation column in the form of vapour; the vapour free of heavy components accesses the rectifier, where regenerated oil is recovered in the lower part thereof and the light components are collected in the upper part thereof, which are condensed in the air condenser and conveyed to a backflow tank; the regenerated oil is circulated back to the solar thermal facility by a first pump; and the light components are recirculated from the backflow tank to the rectifier by a second pump. 